The present invention relates to light-emitting diode (LED) displays and, more specifically, to pixel designs for LED displays.
Large full-color displays for viewing video images in daylight have become increasingly common in the United States and Japan, for example, in sports stadiums and for advertisements on buildings. These large displays comprise hundreds of thousands of pixels that can be arranged in a similar fashion as those in conventional color televisions. Each pixel includes a red, green, and blue light-emitting diode (LED), because they can in combination produce all hues. The three colored diodes in a pixel comprise a triad and are situated in close proximity to each other in a single package, the triad-package. This close proximity ensures that they are not separately visible to viewers, who see only the hue the light-emitting diodes (LEDs) produce in combination. In prior art designs such as those manufactured by Nichia, each triad has a single bullet-lens associated therewith. The three different colored LEDs in an individual triad each emit radiation from a front face and from side faces, but typically with differing proportions of light therefrom. Emission is usually maximal when viewed at a direction normal, or nearly so, to the front face. This emission progressively diminishes when viewed from increasing angles away from said normal. Emission from at least two (typically the green and blue) of the three LEDs, however, falls off at a different rate from the other (i.e., the red). Typically, the red LED will be relatively brighter than the blue and green when viewed at an angle away from the normal. As a result, the display may provide proper color-rendition for a viewer located along the triad axis, but will have different hues for viewers situated at other angles.
Also, although the LED""s radiate in a wide range of directions, the displays are typically viewed by an audience that is located within a vertically confined region, such that the audience views the displays from a limited range of vertical angles. Light emitted by the LED display at angles outside this range is wasted, since it does not reach any viewers. In addition, the display should have the same brightness at all viewing angles, a property of Lambertian emission. The display""s luminous efficiency is maximized by Lambertian emission of the triads. The bullet-lens configuration of the prior art, however, is not an Lambertian emitter and instead provides reduced or excess off-angle brightness.
Thus, there is a need for an LED pixel comprising colored LEDs that fall off at substantially identical rates and that directs substantially all emission toward the viewing audience, in a Lambertian pattern of constant brightness whereby intensity falls off with the cosine of the viewing angle.
In one aspect of the invention, a color display comprises a plurality of pixels, each pixel comprising a plurality of light sources. Each of the light sources comprises a solid-state emitter that emits light of a color different from the other light sources. In addition, each of the light sources includes optics such that intensity variations with respect to view-angle are substantially the same as those of the other light sources.
In another aspect of the invention, a color display comprises a plurality of pixels each comprising a plurality of light sources, wherein the light sources each comprise a light-emitting diode and associated beam-shaping optics.
In yet another aspect of the invention, a color display comprises a plurality of pixels each comprising a plurality of substantially Lambertian sources. Each Lambertian source comprises a light-emitting diode that emits light of a color different from other of the Lambertian sources in the pixel. The Lambertian sources further include optics situated to receive light from the light-emitting diode and configured to redirect the light so as to produce the requisite Lambertian emission pattern.
In still another aspect of the invention, an apparatus comprises a light source that emits light for viewing at various angles, with substantially constant brightness. The light source comprises an LED and a Lambertianizing cup. The light-emitting diode is preferably rectangular. It has a front surface, a back surface opposite the front surface, and four side surfaces; the front surface and the four side surfaces emit light. The front surface and the four side surfaces intersect along four upper edges, respectively. The Lambertianizing cup comprises a bottom support surface and four reflecting surfaces, one facing each of the four side surfaces of the light-emitting diode. At least one opposed pair of the reflector surfaces is configured to provide a substantially cosine fall-off of light intensity for viewing angles in the plane they form with the normal to the front face. In one preferred embodiment, the reflector surfaces are cylindrical and lengthwise parallel to the edges of the front face of the LED. More particularly, the reflector surfaces substantially coincide with a locus of point intersecting an arc that is translated along a line parallel to the upper edge of each of the four sides of the LED. These reflector surfaces are specifically shaped to form non-imaging reflectors.
In another aspect of the invention, a non-imaging optical element comprises first and second reflectors positioned in spaced, facing relationship. Each reflector comprises a middle section and two end sections on opposite sides of the middle section. The middle section comprises a surface in the form of a parabolic cylinder with a line focus. Each end section comprises a surface that joins the surface of the middle section along a parabolic line. The surface of each end section is a paraboloid with a point focus proximate a respective end of the line focus of the middle section. The middle sections of the first and second reflectors form a compound parabolic concentrator trough, which together with the end sections of the reflectors provide beam-shaping optics for restricting beam divergence in one of two orthogonal directions. Preferably, this direction is the vertical direction.
In yet another aspect of the invention, beam-shaping optics for a light-emitting diode comprise first and second sets of reflectors and an optical element comprised of transmitting material. The first set of reflectors is positioned adjacent to sides of the light-emitting diode. The second set of reflectors is oriented to limit beam divergence of light from the light-emitting diode to a limited range of angles in one plane. Preferably, this plane is a vertical plane. The optical element comprised of transmitting material is positioned to receive light from the light-emitting diode and the first and second sets of reflectors. The sets of reflectors and transmitting material are oriented and configured to provide a substantially Lambertian fall-off for light intensity in both planes, but with a limited range of angles in one plane, preferably the vertical and a substantially Lambertian fall-off for light intensity in an orthogonal plane, preferably the horizontal, within a range of angles greater than the limited range of angles.
Another aspect of the invention comprises a method of manipulating an LED""s light to provide Lambertian emission through a requisite range of viewing angles. The method includes the step of collecting light emitted from plural side surfaces of a light-emitting diode and redirecting the light so that it approximates the emission pattern of a single Lambertian surface. The divergence of collected light is reduced in a plane orthogonal to an opposing pair of the plural sides. The collected light is refracted after reducing the divergence.
Still another aspect of the invention comprises a method of shaping the output of a light-emitting diode having a front and a plurality of sides that emit light. This method includes the step of collecting light emitted from the sides and redirecting the light to simulate light produced by a single substantially Lambertian surface. In addition, reflectors are used to reduce the divergence of the beam in one direction, preferably the vertical, to a limited range of angles, while maintaining a substantially Lambertian fall-off within a substantial portion of said limited range.